1. Field of the Invention
The present invention relates to a variable volume pumping HVAC system that uses a common pipe architecture to increase system efficiency and decrease initial system costs as well as decrease operating costs.
2. Background of the Prior Art
For over half a century, virtually all Heating, Ventilation, and Air Conditioning (HVAC) designs that have a large high Pressure drop distribution, such as systems that service a campus or an airport, have used systems which rely on primary-secondary pumping. The primary pumping within the primary or production system, provides constant volume pumping of the relatively low horsepower primary pumps through the chillers, boilers or heat exchangers. The lower horsepower of the primary pumps relative to the secondary pumps is due to the fact that the primary pumps need only to overcome the frictional loss associated with the chillers, boilers, heat exchangers, pipes and valves found in the primary loop. The secondary pumps within the secondary or distribution system, require a much higher horsepower as these pumps must overcome the frictional loss within the secondary loop due to the distribution piping, the fittings, the valves, the coils, etc. Accordingly, these secondary pumps normally are not operating efficiently. To overcome this efficiency problem, several attempts have been made including having the distribution system operate at variable volumes and use either constant speed pumps that ride the pump curves or incorporate variable frequency drives.
In such variable volume pumping systems, three key areas are of critical concern to the system designer, the common or decouple pipe, the chiller, boiler, or heat exchanger sequencing, and the temperature and flow requirements at the point of use.
The common pipe hydraulically decouples the primary pumps from the secondary pumps while allowing thermal interaction. Current design criteria assumes that the largest flow in the primary loop is passing through the common pipe and the maximum pressure drop across the common pipe should not exceed 1.5 ft. The theory is higher friction loss in the common pipe tends to make the primary pumps and the secondary pumps act in series which results in an induced flow in the system. In order to eliminate mixing due to excessive return velocity in the secondary return piping, three pipe diameters of separation are maintained between the secondary supply tee and the secondary return tee. A longer length pipe may result in a pressure drop greater than 1.5 ft.
Proper chiller, boiler and heat exchanger sequencing is another critical aspect of the primary-secondary pumping system in order to maintain overall system efficiency while allowing for proper flows and temperatures at the point of use. The chillers operate in one of three different flow conditions. The first flow condition is where the flow of the secondary system equals the flow of the primary system. In such conditions, which rarely ever occur, the flow coming out of the secondary system is equal to the flow intake of the primary system which results in a thermal balance and no flow in the common pipe.
The second flow condition occurs when the flow coming out of the secondary system exceeds the flow intake of the primary system. In such a condition, the increased load placed on the system results in the secondary pumps requiring more flow than is being produced by the fixed flow of the balanced chillers. As a result the secondary pumps run out on their curves due to the reduction in pressure. The excess flow required by the secondary system must flow through the common pipe. As a result, the water running through the common pipe blends with the supply water leaving the primary pumps and results in a higher water temperature (in an air conditioning mode) than what the chiller produces, which can result in a loss of humidity control. In order to combat this, a another chiller can be brought online in order increase the flow out of the primary system in order to eliminate the blending of the return water with the supply water. Another option is to reset the temperature of the currently operating chiller temperatures to a lower temperature so that the supply water leaving the primary system is at a lower temperature in order to compensate for the blending with the higher temperature return water coming through the common pipe. Each of these options, while addressing the problem at hand, resulting in lower overall system efficiency and higher operating costs.
The third flow condition occurs when the flow from the production system exceeds the flow passing through the distribution system. As the secondary system is pumping at its desired load, the excess flow being produced by the primary system is feed back through the common pipe where it is mixed with the return water supply, resulting in a lowering of the return water supply temperature. While this is desired, improper mixing in the common pipe can lead to excessive unloading and short cycling of the chillers, boilers and heat exchangers, creating excessive starts and thereby shorten the life span of the equipment.
Therefore, it is incumbent on the designer of the primary-secondary pumping system to select chillers in appropriate sizes and to sequence their staging and destaging in order to as closely balance primary and secondary flow as possible. Although this will result in higher operating efficiency and point of use performance, it is not an easy task.
Another critical design feature of the primary-secondary pumping system are the coil selections and control valves and actuators whose functions are to vary the flow properly through the water coils under a variety of system load conditions. If an undersized valve is selected, insufficient flow capacity will be achieved while an oversized valve will produce poor system control.
In reality, many HVAC systems engineers oversize the boilers and chillers of the system as well as the associated pumps in order to be assured of a system that can handle system load requirements. Alternately, many engineers undersize equipment and try to operate at higher Delta Ts and sequence the systems to operate at less than peak performance in order to achieve acceptable system response. In either scenario, either the cost of the system is excessive, or the operating costs are excessive relative to system requirements.
Therefore, in any primary-secondary pumping systems, great care must be taken in the architecture of the thermal plants and to achieve appropriate sequencing of the plants based on design temperatures and flow requirements of the system. System criteria should include, energy efficiency of the system, flow control of the system, system costs and minimization of system component wear and tear. In most designs, tradeoffs are made between the competing system criteria. Even if the systems are designed and configured to existing accepted design criteria, they tend not to operate at peek efficiency due to inherent problems in the current state of the art systems.
Many efficiency problems associated with primary-secondary pumping systems can be traced back to the common or decouple pipe. Current common pipe designs require that the maximum pressure drop not exceed 1.5 ft, which pressure drop is established by assuming that the flow of the largest chiller pump is passing through the common pipe. The diameter of the common pipe being the same as the diameters of the supply line and the return line as well as the relatively short length of the common pipe help maintain this relatively low pressure. The problem occurs during low and medium flow conditions. The present criteria recommends the architecture of the common pipe, the first opening from the secondary loop, flow to the suction side of the secondary pump. In such conditions, water flows in both directions in the common pipe at the same time resulting in no thermal interaction within the common pipe. As the water traveling in the pipe travels in a swirl, and as water is traveling in both directions within the pipe, due to the relatively low pressure drop across the pipe and the relatively low flow, thermal interaction only occurs at the point where both spirals touch, which results in water in the primary loop returning to the equipment at near the same temperature as the leaving temperature. This artificially unloads the equipment and a higher than desired temperature feeding the secondary loop. This causes the control valves to open to try to increase flow to the coils, which starts a cycle of excess flow, which in turn overloads the equipment, and the cycle starts all over again.
Therefore, there exists a need in the art for a primary-secondary pumping HVAC system that addresses the above-stated needs in the art. Specifically, such a system must simplify the proper selection of the appropriate sized boilers and chillers as well as the primary and secondary system pumps without the need to oversize them or to operate them at less than peak operating conditions in order to help assure a proper load distribution from the system. Such a system must be easy to design and easy to sequence and operate while tracking proper system load distribution in all loading environments.